Overrunning, backstop or one-way clutches describe clutches in which the output locks to the input when the input is rotated in one direction (i.e., the power stroke) and free-wheels when the input shaft is rotated in the opposite direction (i.e., the non-power stroke). A transmission that employs a continuously oscillating drive mechanism with which this type of clutch may be used is the Continuously Variable Ratio Transmission disclosed in U.S. Pat. No. 5,674,144 ("the '144 patent"), hereby incorporated herein by reference. There are primarily three types of these overrunning clutches, and each has disadvantages as described below.
A roller ramp clutch has compartmentalized ball or roller bearings placed around the input shaft. The compartment, being shaped like a ramp relative to the circumference of the shaft, is closer to the shaft at one end of the ramp and rises away from the shaft. The ramp acts as the inner race for the clutch. The outer race is a cylindrical unit that fits around the bearings, ramps and shaft. The clearance between the inside of this outer race and the bearings is small enough to allow the bearings to turn freely when they are at the end of the ramp closest to the input shaft. As a bearing rolls up its ramp and therefore moves further from the input shaft, the bearing comes into contact with the inner surface of the outer race and wedges between the ramp of the inner race and the surface of the outer race. Typically, a spring is used to bias the bearings toward the compression area between the ramp and the inner surface of the outer race.
In operation, as the shaft rotates in one direction, the bearings roll toward the end of the ramp closest to the shaft, where they are allowed to freely turn and the outer race free-wheels. When the input shaft rotation is reversed, the spring forces the bearings to make contact with the inner surface of the outer race, and the reversed rotary motion causes them to roll toward the compression end of the ramp. Once the bearings are wedged into the space between the ramp and inner surface of the outer race, they cannot rotate and they lock the outer race to the inner race. There can be as many as thirty or more parts in the typical roller ramp clutch.
Another type of overruning clutch, the sprag clutch, has an inner and outer race with the surfaces thereof being concentric with the input shaft. Sprags are placed in the area between these two surfaces. These sprags are metal blocks that are formed into a shape somewhat like a warped figure eight which, in cross section, is narrower when the clutch is freewheeling. When the rotation direction of the input shaft is reversed, these sprags pivot and wedge their larger cross-sections between the inner and outer race, providing lock-up. These sprags must butt up against each other around the entire circumference of the inner race, yet be allowed to pivot freely to lock-up. In most cases, a cage must be provided to keep constant alignment between the sprags, and there must be a spring surrounding the sprag assembly to keep equal tension on all of the sprags.
Both the roller ramp clutches and the sprag clutches require the wedging action necessary for lock-up to exert force primarily in an outward direction. This can cause distortion of the outer race if it is not of sufficient mass to remain rigid. Also in this regard with respect to the sprag clutches, high operating speeds on the sprag clutch and the resulting outward centrifugal force generated, may cause the sprags to lift off of the inner race, which prohibits lock-up. A higher spring biasing force must be used to compensate for this, but wear from the surfaces sliding against each other is increased, thereby limiting the service life.
Both of these types of clutches have very little actual surface contact between the bearings or sprags and the inner and outer races. This limited contact area reduces the capability of the clutch to transmit the torque from the input shaft to the outer race once the amount of friction achieved through this limited contact area is surpassed.
Additionally, for consistent operation of either of these types of clutches over multiple cycles, all bearings or sprags must engage at exactly the same time, which is difficult to achieve. Under high torque loads, if one bearing or sprag locks up before the others, failure occurs.
The third type of overruning clutch is a family of devices which all provide friction by compression. One of these devices is the conventional screw cone clutch. It includes an output cone rotatably mounted onto the input shaft and against a stop which maintains the output cone's placement on the shaft. The input cone is screwed onto the input shaft in a mating relationship with the output cone.
In operation, when locked up, the input cone is compressed against and rotates at the same speed as the output cone until the rotation direction of the input shaft is reversed. With the shaft's change in direction, the input cone is screwed away from the output cone. The input cone continues to rotate in the same direction as the output cone, however, until the friction from the output cone falls below the friction imparted on the input cone by the reverse rotation of the input shaft. It is the balance between these two forces that determines the clearance between the input and output cones. The inertia carried by the input cone moves it further away from the output cone that the final clearance distance. The torque required to change the direction of the input cone also tends to cause the input cone to move further away from the output cone until the two friction sources are balanced. At some rate of operation, the screw cone will not change its direction of rotation with the input shaft. Thus, the input cone must be rotated forward several additional degrees than a re normally necessary to achieve lock-up, resulting in additional rotation of the input shaft. At high speeds, conventional screw cone clutches may miss cycles resulting in decreased or no lock-up.